Trench capacitor with spacer-less fabrication process

ABSTRACT

A trench capacitor and method of fabrication are disclosed. The SOI region is doped such that a selective isotropic etch used for trench widening does not cause appreciable pullback of the SOI region, and no spacers are needed in the upper portion of the trench.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor fabrication,and more particularly, to trench capacitors, and methods of fabrication.

BACKGROUND OF THE INVENTION

A DRAM cell is essentially a capacitor for storing charge and a passtransistor (also called a pass gate or access transistor) fortransferring charge to and from the capacitor. Data stored in the cellis determined by the absence or presence of charge on the storagecapacitor. Because cell size affects chip density, and cost, reducingcell area is one of the DRAM designer's primary goals.

One way to accomplish this density goal without sacrificing storagecapacitance is to use trench capacitors in the cells. Trench capacitorscan be formed by etching deep trenches in a silicon wafer and formingvertically oriented capacitors within each deep trench. Thus, thesurface area required for the storage capacitor is dramatically reducedwithout sacrificing capacitance, and correspondingly, storable charge.

Trench-type memory devices are advantageous, in comparison to planarmemory configurations, for increased density, performance andlithographic considerations. Trench-type memory devices increase densityby reducing the cell area of each memory device, therefore allowing forcloser positioning of adjacent memory devices.

As the trend towards miniaturization increases, so does the performancedemands of electronic devices, it is desirable to have an improved DRAMand method of fabrication.

SUMMARY

In one embodiment, a method for fabricating a trench in a semiconductorstructure is provided. The semiconductor structure comprises a substrateand a layer stack, with the layer stack comprising asilicon-on-insulator layer. The method comprises performing a firstanisotropic etch into the semiconductor structure, wherein the firstanisotropic etch traverses the layer stack and penetrates the substrate,thereby forming an upper trench portion, performing an angular implantin the upper trench portion, thereby implanting a trench contact regionof the silicon-on-insulator layer, performing a second anisotropic etchinto the semiconductor structure, extending the upper trench portionfurther into the substrate, thereby forming a lower trench portion, andperforming a selective isotropic etch, thereby widening the lower trenchportion.

In another embodiment, a method for fabricating a trench in asemiconductor structure is provided. The semiconductor structurecomprises a substrate and a layer stack, with the layer stack comprisinga silicon-on-insulator layer. The method comprises performing a firstanisotropic etch into the semiconductor structure, wherein the firstanisotropic etch traverses the silicon-on-insulator layer, and partiallytraverses the BOX layer, thereby forming an upper trench portion,performing an implant in the upper trench portion, thereby implanting atrench contact region of the silicon-on-insulator layer, performing asecond anisotropic etch into the semiconductor structure, completelytraversing the BOX layer, and extending the upper trench portion intothe substrate, performing a third anisotropic etch into thesemiconductor structure, extending the upper trench portion further intothe substrate, thereby forming a lower trench portion, and performing aselective isotropic etch, thereby widening the lower trench portion.

In another embodiment, a semiconductor structure is provided. Thestructure comprises a substrate, an insulator layer disposed on thesubstrate, a silicon-on-insulator layer disposed on the insulator layer,a trench formed in the substrate, wherein an upper portion of the trenchtraverses the insulator layer and silicon-on-insulator layer, and alower portion of the trench is formed within the substrate, and whereinthe upper portion has a first width and the lower portion has a secondwidth, and wherein the second width is greater than the first width.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure, operation, and advantages of the present invention willbecome further apparent upon consideration of the following descriptiontaken in conjunction with the accompanying figures (FIGs.). The figuresare intended to be illustrative, not limiting.

Certain elements in some of the figures may be omitted, or illustratednot-to-scale, for illustrative clarity. The cross-sectional views may bein the form of “slices”, or “near-sighted” cross-sectional views,omitting certain background lines which would otherwise be visible in a“true” cross-sectional view, for illustrative clarity.

Often, similar elements may be referred to by similar numbers in variousfigures (FIGs) of the drawing, in which case typically the last twosignificant digits may be the same, the most significant digit being thenumber of the drawing figure (FIG).

FIG. 1 shows a prior art trench capacitor.

FIG. 2 shows a semiconductor structure at a starting point forfabrication of a trench capacitor in accordance with an embodiment ofthe present invention.

FIG. 3 shows a subsequent step in the fabrication process, afterperforming a first anisotropic etch.

FIG. 4 shows a subsequent step in the fabrication process of performingan angular upper trench implant.

FIG. 5 shows a subsequent step in the fabrication process, afterperforming a second anisotropic etch.

FIG. 6 shows a subsequent step in the fabrication process, afterperforming a selective isotropic etch.

FIG. 7 shows an alternative embodiment of a subsequent step in thefabrication process, following from FIG. 2, after performing a firstanisotropic etch.

FIG. 8 shows a subsequent step in the fabrication process, followingfrom FIG. 7, of performing an upper trench implant.

FIG. 9 shows a subsequent step in the fabrication process, followingfrom FIG. 8, after performing a second anisotropic etch.

FIG. 10 shows a trench capacitor in accordance with an embodiment of thepresent invention.

FIG. 11 is a flowchart indicating process steps for an embodiment of thepresent invention.

FIG. 12 shows a block diagram of an exemplary design flow.

DETAILED DESCRIPTION

To provide context for the detailed description, a prior artsemiconductor structure 100 shown in FIG. 1 is briefly described below.A silicon substrate 102 has an insulator layer comprised of buried oxide(BOX layer) 104 disposed on the substrate 102. Above the BOX layer 104is a silicon-on-insulator (SOI) layer 106. Above the SOI layer is anitride cap layer 107. Above the nitride cap layer 107 is a hard masklayer 108. The hard mask layer 108, nitride cap layer 107, SOI layer106, and BOX layer 104 comprise a layer stack 119 that is disposed ontop of substrate 102.

A trench 109 is formed within the semiconductor structure. The trench109 comprises an upper trench portion 112, and a lower trench portion114. The lower trench portion 114 is wider than the upper trench portion112. Nitride spacer 110 lines the upper portion 112 of trench 109. Thisreduces the critical dimension (width of the upper trench portion) whichcomplicates anisotropic etching, such as is performed with a reactiveion etch (RIE) technique. The existing spacer 110 complicates theprocess of making a deeper trench with a similar aspect ratio. Thepresence of spacer 110 also makes the effective size of the trenchopening smaller during implantation process used to form a buried plate.Fabrication of a buried plate depends on a certain level of implanteddopants through the top DT (deep trench) opening after DT RIE (reactiveion etch) to form the common bottom electrode, i.e. buried plate for aDRAM. It is therefore desirable to fabricate a trench capacitor withoutsuch a spacer. However, the spacer 110 performs an important function ofprotecting the SOI layer 106 during subsequent etching. Embodiments ofthe present invention provide for a trench capacitor and method offabrication that eliminates the spacer without compromising the SOIlayer 106.

FIG. 2 shows a semiconductor structure 200 at a starting point forfabrication of a trench capacitor in accordance with an embodiment ofthe present invention. As stated previously, often, similar elements maybe referred to by similar numbers in various figures (FIGs) of thedrawing, in which case typically the last two significant digits may bethe same, the most significant digit being the number of the drawingfigure (FIG). Hence, silicon substrate 202 of FIG. 2 is similar tosilicon substrate 102 of FIG. 1, and BOX layer 204 of FIG. 2 is similarto BOX layer 104 of FIG. 1, for example.

FIG. 3 shows a semiconductor structure 300 after a subsequent step inthe fabrication process, after performing a first anisotropic etch. Theupper portion 312 of trench 309 is formed by performing an anisotropicetch to a depth D1, such that upper portion 312 traverses the layerstack 319, penetrating through BOX layer 304 and into substrate 302. Inone embodiment, BOX layer 304 has a thickness in the range of 40nanometers to 160 nanometers.

FIG. 4 shows a semiconductor structure 400 after a subsequent step inthe fabrication process of performing an angular upper trench implant.The angular implanting, represented by lines “T” are at angle “A” fromhorizontal. In one embodiment, angle A is in the range of 15 to 60degrees. The angled implant provides dopants to trench contact region415 of SOI layer 406. However, the angled implant does not reach thebase 413 of the upper portion 412 of the trench. Hence, substrate 402does not receive dopants from the angled implant. The dopant is oppositeto the type of substrate 402. For example, if substrate 402 is comprisedof N-type silicon, then the dopant used for the angular implanting iscomprised of a P-type dopant, such as boron. Similarly, if substrate 402is comprised of P-type silicon, then the dopant used for the angularimplanting is comprised of an N-type dopant, such as arsenic orphosphorus. In one embodiment, the dopant concentration ranges fromabout 1E-19 to about 1E-17 atoms/cm3. In one embodiment, SOI layer 406has a thickness in the range of 30 nanometers to 110 nanometers.

FIG. 5 shows a semiconductor structure 500 after a subsequent step inthe fabrication process, after performing a second anisotropic etch,extending trench 509 to a depth D2.

FIG. 6 shows a semiconductor structure 600 after a subsequent step inthe fabrication process, after performing a selective isotropic etch.The selective isotropic etch may be a wet etch. The selective isotropicetch widens the lower portion 614 of trench 609 to a width W2, which isgreater than width W1 of the upper portion 612 of trench 609. The largerwidth W2 provides additional surface area, which in turn allows forincreased capacitance. In one embodiment, width W1 ranges from 40nanometers to 120 nanometers, and width W2 ranges from 90 nanometers to180 nanometers.

The selective isotropic etch does not significantly etch the SOI layer606 because of the dopants in trench contact region 615. In oneembodiment, a selective wet etch is used, and the etchant used for theselective wet etch is HF. In one embodiment, a highly selectiveKOH-based etchant is used for a boron-doped SOI layer.

FIG. 7 shows a semiconductor structure 700 of an alternative embodiment,after a subsequent step in the fabrication process, following from FIG.2, after performing a first anisotropic etch. The upper portion 712 oftrench 709 is formed by performing an anisotropic etch to a depth D3,such that upper portion 712 does not completely traverse layer stack719, and instead, stops part way through BOX layer 704, only partiallytraversing BOX layer 704, and does not penetrate into substrate 702(compare with 312 of FIG. 3).

FIG. 8 shows a semiconductor structure 800 after a subsequent step inthe fabrication process, following from FIG. 7, of performing an uppertrench implant. The upper trench implant, represented by lines U, may bea vertical implant. Scattering of dopants that occurs during thisprocess implants dopants into trench contact region 815 of SOI layer806. Because the substrate 802 is not exposed, the dopants do not reachinto the substrate 802, and hence, the angled implant described in FIG.4 is not essential here.

FIG. 9 shows a semiconductor structure 900 after a subsequent step inthe fabrication process, following from FIG. 8, after performing asecond anisotropic etch. The trench 909 is now extended to a depth D4,and now traverses layer stack 919, penetrating the BOX layer 904 andextending into substrate 902. From this point forward, the selectiveisotropic etch technique shown in FIG. 6 is used to achieve the widenedlower portion of the trench capacitor (see 614 of FIG. 6).

FIG. 10 shows a trench capacitor in accordance with an embodiment of thepresent invention. Proceeding forward from structure 600 shown in FIG.6, standard fabrication techniques are used to complete the trenchcapacitor. The resulting trench capacitor 1000 does not have any spacers(compare with spacers 110 of FIG. 1). Trench contact region 1015 is partof SOI layer 1006, and forms part of the trench wall. Trench contactregion 1015 is substantially flush with the other layers (1004, 1007,1008) along the trench. Hence, there is negligible “pull back” of theSOI layer 1006, and the spacer, and its disadvantages are avoided.

A metal plate 1024, dielectric layer 1026, and conductive plug 1028 aredeposited with in the trench (609 of FIG. 6) to form a functional trenchcapacitor. In one embodiment, the outer electrode plate is comprised ofheavily doped silicon or a silicide based on TiN, tungsten, copper,cobalt, or nickel. The dielectric layer 1026 may be comprised of hafniumoxide. The conductive plug may be comprised of a metal, such as TiN,tungsten or copper, or may be comprised of polysilicon.

FIG. 11 is a flowchart indicating process steps for embodiments of thepresent invention. One embodiment starts with process step 1150, whichcomprises performing a first anisotropic etch. This etch penetrates theBOX layer and extends into the substrate (see FIG. 3). The process thenproceeds to process step 1152, where an angular implant is performed.This implants the trench contact region (region of the SOI that is alongthe trench wall, see FIG. 4). The process then proceeds to process step1158, where a second anisotropic etch is performed, to create the lowertrench portion (See FIG. 5). Then, in process step 1164, a selectiveisotropic etch is performed to widen the lower portion of the trench(See FIG. 6).

An alternative embodiment is indicated in FIG. 11 with the followingprocess steps. In process step 1154, a first anisotropic etch isperformed. In this embodiment, the first anisotropic etch stops part waythrough the BOX layer (see FIG. 7). In process step 1156, a verticalimplant is performed. Since the BOX layer is not completely penetrated,the underlying substrate is not implanted (see FIG. 8). In process step1160, a second anisotropic etch is performed to completely penetrate theBOX layer (see FIG. 9). In process step 1162, a third anisotropic etchis performed to etch the substrate to form the lower trench. Theresulting semiconductor structure is similar to that for step 1158 (seeFIG. 5). Next, the process proceeds to step 1164 described previously,to form the widened lower trench portion (see FIG. 6).

FIG. 12 shows a block diagram of an exemplary design flow 1600 used forexample, in semiconductor IC logic design, simulation, test, layout, andmanufacture. Design flow 1600 includes processes, machines and/ormechanisms for processing design structures or devices to generatelogically or otherwise functionally equivalent representations of thedesign structures and/or devices described above and shown in FIGS.3-11. The design structures processed and/or generated by design flow1600 may be encoded on machine-readable transmission or storage media toinclude data and/or instructions that when executed or otherwiseprocessed on a data processing system generate a logically,structurally, mechanically, or otherwise functionally equivalentrepresentation of hardware components, circuits, devices, or systems.Machines include, but are not limited to, any machine used in an ICdesign process, such as designing, manufacturing, or simulating acircuit, component, device, or system. For example, machines mayinclude: lithography machines, machines and/or equipment for generatingmasks (e.g. e-beam writers), computers or equipment for simulatingdesign structures, any apparatus used in the manufacturing or testprocess, or any machines for programming functionally equivalentrepresentations of the design structures into any medium (e.g. a machinefor programming a programmable gate array).

Design flow 1600 may vary depending on the type of representation beingdesigned. For example, a design flow 1600 for building an applicationspecific IC (ASIC) may differ from a design flow 1600 for designing astandard component or from a design flow 1600 for instantiating thedesign into a programmable array, for example a programmable gate array(PGA) or a field programmable gate array (FPGA) offered by Altera® Inc.or Xilinx® Inc.

FIG. 12 illustrates multiple such design structures including an inputdesign structure 1620 that is preferably processed by a design process1610. Design structure 1620 may be a logical simulation design structuregenerated and processed by design process 1610 to produce a logicallyequivalent functional representation of a hardware device. Designstructure 1620 may also or alternatively comprise data and/or programinstructions that when processed by design process 1610, generate afunctional representation of the physical structure of a hardwaredevice. Whether representing functional and/or structural designfeatures, design structure 1620 may be generated using electroniccomputer-aided design (ECAD) such as implemented by a coredeveloper/designer. When encoded on a machine-readable datatransmission, gate array, or storage medium, design structure 1620 maybe accessed and processed by one or more hardware and/or softwaremodules within design process 1610 to simulate or otherwise functionallyrepresent an electronic component, circuit, electronic or logic module,apparatus, device, or system such as those shown in FIGS. 3-11. As such,design structure 1620 may comprise files or other data structuresincluding human and/or machine-readable source code, compiledstructures, and computer-executable code structures that when processedby a design or simulation data processing system, functionally simulateor otherwise represent circuits or other levels of hardware logicdesign. Such data structures may include hardware-description language(HDL) design entities or other data structures conforming to and/orcompatible with lower-level HDL design languages such as Verilog andVHDL, and/or higher level design languages such as C or C++.

Design process 1610 preferably employs and incorporates hardware and/orsoftware modules for synthesizing, translating, or otherwise processinga design/simulation functional equivalent of the components, circuits,devices, or logic structures shown in FIGS. 3-11 to generate a Netlist1680 which may contain design structures such as design structure 1620.Netlist 1680 may comprise, for example, compiled or otherwise processeddata structures representing a list of wires, discrete components, logicgates, control circuits, I/O devices, models, etc. that describes theconnections to other elements and circuits in an integrated circuitdesign. Netlist 1680 may be synthesized using an iterative process inwhich netlist 1680 is resynthesized one or more times depending ondesign specifications and parameters for the device. As with otherdesign structure types described herein, netlist 1680 may be recorded ona machine-readable data storage medium or programmed into a programmablegate array. The medium may be a non-volatile storage medium such as amagnetic or optical disk drive, a programmable gate array, a compactflash, or other flash memory. Additionally, or in the alternative, themedium may be a system or cache memory, buffer space, or electrically oroptically conductive devices and materials on which data packets may betransmitted and intermediately stored via the Internet, or othernetworking suitable means.

Design process 1610 may include using a variety of inputs; for example,inputs from library elements 1630 which may house a set of commonly usedelements, circuits, and devices, including models, layouts, and symbolicrepresentations, for a given manufacturing technology (e.g., differenttechnology nodes, 32 nm, 45 nm, 90 nm, etc.), design specifications1640, characterization data 1650, verification data 1660, design rules1670, and test data files 1685 (which may include test patterns andother testing information). Design process 1610 may further include, forexample, standard circuit design processes such as timing analysis,verification, design rule checking, place and route operations, etc. Oneof ordinary skill in the art of integrated circuit design can appreciatethe extent of possible electronic design automation tools andapplications used in design process 1610 without deviating from thescope and spirit of the invention. The design structure of the inventionis not limited to any specific design flow.

Design process 1610 preferably translates an embodiment of the inventionas shown in FIGS. 3-11, along with any additional integrated circuitdesign or data (if applicable), into a second design structure 1690.Design structure 1690 resides on a storage medium in a data format usedfor the exchange of layout data of integrated circuits (e.g. informationstored in a GDSII (GDS2), GL1, OASIS, or any other suitable format forstoring such design structures). Design structure 1690 may compriseinformation such as, for example, test data files, design content files,manufacturing data, layout parameters, wires, levels of metal, vias,shapes, data for routing through the manufacturing line, and any otherdata required by a semiconductor manufacturer to produce an embodimentof the invention as described above with reference to FIGS. 3-11. Designstructure 1690 may then proceed to a stage 1695 where, for example,design structure 1690: proceeds to tape-out, is released tomanufacturing, is released to a mask house, is sent to another designhouse, is sent back to the customer, etc.

Although the invention has been shown and described with respect to acertain preferred embodiment or embodiments, certain equivalentalterations and modifications will occur to others skilled in the artupon the reading and understanding of this specification and the annexeddrawings. In particular regard to the various functions performed by theabove described components (assemblies, devices, circuits, etc.) theterms (including a reference to a “means”) used to describe suchcomponents are intended to correspond, unless otherwise indicated, toany component which performs the specified function of the describedcomponent (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein illustrated exemplary embodiments of theinvention. In addition, while a particular feature of the invention mayhave been disclosed with respect to only one of several embodiments,such feature may be combined with one or more features of the otherembodiments as may be desired and advantageous for any given orparticular application.

The invention claimed is:
 1. A method for fabricating a trench in asemiconductor structure comprising a substrate and a layer stack, thelayer stack comprising a silicon-on-insulator layer, the methodcomprising: performing a first anisotropic etch into the semiconductorstructure, wherein the first anisotropic etch traverses the layer stackand penetrates the substrate, thereby forming an upper trench portion;implanting a region of the silicon-on-insulator layer with an angularimplant to form a symmetrical trench contact region; performing a secondanisotropic etch into the semiconductor structure, extending the uppertrench portion further into the substrate, thereby forming a lowertrench portion; and performing a selective isotropic etch, therebywidening the lower trench portion.
 2. The method of claim 1, whereinperforming an angular implant comprises implanting at an angle rangingfrom about 15 degrees, to about 60 degrees.
 3. The method of claim 1,wherein performing an angular implant comprises implanting dopantscomprising a material selected from the group consisting of arsenic,phosphorous, and boron.
 4. The method of claim 1, wherein performing anangular implant comprises implanting dopants with a concentrationranging from about 1E-19 atoms/cm3 to about 1E-17 atoms/cm3.
 5. Themethod of claim 1, wherein performing a selective isotropic etchcomprises performing a wet etch.
 6. The method of claim 5, whereinperforming a wet etch is performed with an etchant of HF.
 7. The methodof claim 1, wherein performing a second anisotropic etch is performedwith a reactive ion etch.